There is known a boring head for facing operations (cf. B. M. Bromberg et al., Almazno-rastochnye stanki /Diamond Boring Machines/, Machinostronie Publishers, Moscow, 1965, p. 83), comprising a housing which is kinematically coupled to a spindle and accommodates a counterbalance and a carriage carrying a mandrel for a cutting tool. The housing also accommodates a rod coaxial with the former and kinematically coupled to a drive which moves the rod in the axial direction. The rod is constructed as a frame interacting with the carriage and counterbalance. The carriage and counterbalance are radially movable in the opposite directions by means of helical racks provided on the lateral sides of the rod and meshed with corresponding helical racks provided on the carriage and counterbalance. The helical racks are such that the carriage and counterbalance cover equal distances as they move in opposite directions with their masses being equal. As the spindle is set in rotation, the carriage and counter-balance are acted upon by their own centrifugal forces which also act on the rod, producing a twisting moment. The latter causes some deformation of the rod and a certain amount of play which affects the machining accuracy. In addition, the boring head in question is disadvantageous in its relatively great weight and size due to the presence of the counterbalance received in a special groove provided in the housing.
There is also known a boring head of a multiple-operation machine tool, which is the prototype of the present invention. This boring head comprises a housing kinematically coupled to a spindle and adapted to accommodate a carriage which carries a mandrel for a cutting tool and has three slanted surfaces. One of these interacts with a slanted surface of a rod arranged in the housing coaxially with the latter and kinematically coupled to a drive which moves it in the axial direction. The two other slanted surfaces are symmetrically arranged in relation to the axis of rotation of the boring head and interact with rollers whose axles are perpendicular to the axis of rotation of the boring head and secured at the ends of a fork spring-loaded against the carriage. The fork is coaxial with the housing. The rollers interact with the slanted surfaces on lines found in a plane extending through the axis of rotation of the boring head (cf. USSR Inventor's Certificate No. 665,994). The carriage is displaced in relation to the axis of the boring head. As the boring head rotates, a centrifugal force is produced, which is directed at a perpendicular to the axis of rotation. This force bends the housing of the boring head and the spindle and increases the load on the spindle bearings. At a low spindle speed, this force does not tangibly affect the accuracy of machining, but a low spindle speed means a low efficiency of the machine tool. Raising the spindle speed in order to raise the efficiency produces a centrifugal force which bends the housing and the spindle and causes contact deformations in the spindle bearings. These factors affect the accuracy with regard to the diameter of machined holes and make them assymetrical due to a non-uniform rigidity of the spindle bearings in different directions relative to the workpiece.